With the rapid development of robot technologies, new requirements are put forward for the performance such as transmission precision, reliability and maintenance-free of transmission units and transmission systems thereof. Existing mowers generally have a wheel and a drive motor for driving the wheel to rotate, and an output end of the drive motor is connected with a rotating shaft of the wheel to drive the wheel to rotate; however, mowers of this structure have defects as follows: 1. The drive motor is located outside the wheel, and a position needs to be designed on the mower to mount the drive motor, thus the structure is relatively complex. 2. The drive motor occupying certain space impacts the setting of other mechanisms on the mower and thus reduces the transmission precision and reliability. 3. Common inner rotor motors have high rotation speed but relatively low movement inertia.